1. Field of the Invention
The present invention relates to an ink jet printing apparatus and more particularly to a constitution for agitating an ink inside an ink tank.
2. Description of the Related Art
An inkjet printer, which is a typical inkjet printing apparatus, ejects an ink on a printing medium such as printing sheets to print images and characters. Among apparatuses of this type, a printer handling larger-sized printing sheets such as A0-size sheets and a professional-use printer for relatively large quantities of printing require a large-capacity ink tank in accordance with relatively large ink consumption. Further, a multi-color printer using various types of inks requires many tanks which accommodate inks according to the number of colors thereof. The above-described printers greater in ink consumption are often provided with an ink supply source such as an ink tank at a predetermined position thereof due to the fact that it is inconvenient to move a great-capacity ink tank mounted on a carriage together with a printing head. Therefore, a constitution has been adopted in which an ink is temporarily supplied from the ink supply source via a tube to a sub tank on the carriage and then supplied from the sub tank to the printing head. The sub tank of the above constitution is used to absorb a variation in pressure of the ink in line with movement of the carriage, so as to stabilize supply of the ink to the printing head.
On the other hand, as an ink used for printers, an ink in which a pigment is dispersed (herein after, referred to as a pigment ink) has been known. The pigment ink contains pigments as a coloring agent and in which the pigment is dispersed into an ink solvent. The pigment ink has an advantage that color develops vividly but has a fundamental disadvantage that when it is allowed to stand for a long period of time, for example, over about one to two months, the pigment settles out. When the pigment settles out as described above, an ink inside the sub tank is made thin at the upper part of the tank and made thick at the lower part of the tank, which results in unevenness in concentrations of the pigment. Then, this unevenness in concentrations will provide such a problem that the color of an image changes to a dark color or a light color between an initial stage of printing and a subsequent stage thereof.
Therefore, where a pigment ink is used conventionally, the ink is agitated to remove an unevenness in concentrations of pigment. Japanese Patent Laid-Open No. 2003-159813 has described that, as shown in FIG. 12, a spherical weight 15 which can move is accommodated inside an ink cartridge and the weight moves inside the cartridge in association with motions of the carriage, thereby agitating an ink. Further, Japanese Patent Laid-Open No. 2006-188008 has described a cartridge C in which an ink IN and air EA separated from the ink IN are filled in a state where the ink cartridge is not yet used as shown in Step (a) of FIG. 13. Since the air EA is filled in the cartridge C, in line with movement of the cartridge by the carriage shown in Step (b) to Step (f) of FIG. 13, the ink IN inside the cartridge is given mobility due to the presence of the air EA, thus making it possible to agitate the ink IN. Specifically, a space is formed by the thus filled air EA in the cartridge C, by which the ink IN is mobilized in association with the movement of the cartridge C. This mobility causes a rotational flow in the ink IN and the rotational flow agitates the ink IN.
However, as shown in the agitation constitution described in Japanese Patent Laid-Open No. 2003-159813, in a constitution in which a weight is used to agitate, there is a risk that a moving body may damage an inner wall of an ink cartridge on collision with the inner wall thereof. Further, there are problems that the ink cartridge for accommodating the moving body is made more complicated in structure and constituents for the ink cartridge including the moving body are additionally required leading to an increase in production costs.
The agitation constitution disclosed in Japanese Patent Laid-Open No. 2006-188008 is not made complicated in structure for an ink accommodation portion of an ink tank and also able to give a sufficient agitation effect to an accommodated ink, while suppressing an increase in production costs. However, since a gas is filled inside the ink accommodation portion even in a state where an ink tank is not yet used, the filled air may expand with an increase in external temperatures, by which pressure inside the sub tank may increase and the ink may leak outside the tank.